Add min_issue to covidcast metadata endpoint

[capnrefsmmat]

The endpoint contains max_issue, but does not report min_issue, so users don't know what range they can do as_of queries for. We should add this to the metadata calculation and the API endpoint, then update the clients to ensure they return it in correct format (as a date object rather than a string).

[krivard]

we should revisit this once we have a baseline scale test rig in place.

cc @rzats this could be a good example to use to extend the rig to include DB-centric/non-API performance